The present invention relates to nuclear imaging, and more particularly, to systems, methods, and cameras for radioactive-emission detection and measurements, without coincidence, with sensitivity which meets, and even outperforms that of PET, in terms of speed and spatial resolution, and with a high spectral resolution not available in PET.
Radionuclide imaging aims at obtaining an image of a radioactively labeled substance, that is, a radiopharmaceutical, within the body, following administration, generally, by injection. The substance is chosen so as to be picked up by active pathologies to a different extent from the amount picked up by the surrounding, healthy tissue; in consequence, the pathologies are operative as radioactive-emission sources and may be detected by radioactive-emission imaging. A pathology may appear as a concentrated source of high radiation, that is, a hot region, as may be associated with a tumor, or as a region of low-level radiation, which is nonetheless above the background level, as may be associated with carcinoma.
A reversed situation is similarly possible. Dead tissue has practically no pick up of radiopharmaceuticals, and is thus operative as a cold region.
The mechanism of localization of a radiopharmaceutical in a particular organ of interest depends on various processes in the organ of interest such as antigen-antibody reactions, physical trapping of particles, receptor site binding, removal of intentionally damaged cells from circulation, and transport of a chemical species across a cell membrane and into the cell by a normally operative metabolic process. A summary of the mechanisms of localization by radiopharmaceuticals is found in http://www(dot)lunis(dot)luc(dot)edu/nucmed/tutorial/radpharm/i(dot)htm.
The particular choice of a radionuclide for labeling antibodies depends upon the chemistry of the labeling procedure and the isotope nuclear properties, such as the number of gamma rays emitted, their respective energies, the emission of other particles such as beta or positrons, the isotope half-life, and the decay scheme.
In PET imaging, positron emitting radio-isotopes are used for labeling, and the imaging camera detects coincidence photons, the gamma pair of 0.511 Mev, traveling in opposite directions. Each coincident detection defines a line of sight, along which annihilation takes place. As such, PET imaging collects emission events, which occurred in an imaginary tubular section enclosed by the PET detectors. A gold standard for PET imaging is PET NH3 rest myocardial perfusion imaging with N-13-ammonia (NH3), at a dose level of 740 MBq, with attenuation correction. Yet, since the annihilation gamma is of 0.511 Mev, regardless of the radio-isotope, PET imaging does not provide spectral information, and does not differentiate between radio-isotopes.
In SPECT imaging, primarily gamma emitting radio-isotopes are used for labeling, and the imaging camera is designed to detect the actual gamma emission, generally, in an energy range of approximately 11-511 KeV. Generally, each detecting unit, which represents a single image pixel, has a collimator that defines the solid angle from which radioactive emission events may be detected.
Because PET imaging collects emission events, in the imaginary tubular section enclosed by the PET detectors, while SPECT imaging is limited to the solid collection angles defined by the collimators, generally, PET imaging has a higher sensitivity and spatial resolution than does SPECT. Therefore, the gold standard for spatial and time resolutions in nuclear imaging are defined for PET.
Radiopharmaceuticals are a powerful labeling tool, yet the radiation dose to the patients needs to be taken into account.
In the International System of units (SI), the becquerel (Bq) is the unit of radioactivity. One Bq is 1 disintegration per second (dps). The curie (Ci) is the old standard unit for measuring radioactivity of a given radioactive sample and is equivalent to the activity of 1 gram of radium, originally defined as the amount of material that produces 3.7×1010 dps. Regarding dose levels applicable to radiopharmaceuticals, 1 GBq=27 millicuries.
The rad is a unit of absorbed radiation dose in terms of the energy deposited in a living tissue, and is equal to an absorbed dose of 0.01 joules of energy per kilogram of tissue.
The biologically effective dose in rems is the dose in rads multiplied by a “quality factor” which is an assessment of the effectiveness of that particular type and energy of radiation. Yet, for gamma and beta rays, the quality factor is 1, and rad and rem are equal. For alpha particles, the relative biological effectiveness (rem) may be as high as 20, so that one rad is equivalent to 20 rems.
The recommended maximum doses of radiopharmaceuticals are 5 rems for a whole body dose and 15 rads per organ, while the allowable dose for children is one tenth of the adult level. The per-organ criterion protects organs where accumulation takes place. For example, radiopharmaceuticals for which removal is primarily by the liver should be administered at a lower dose than those for which removal is partly by the liver and partly by the kidney, because in the former, a single organ is involved with the removal, and in the latter, there is sharing of the removal.
In order to minimize exposure to the tissue, radiopharmaceuticals, which have a long half life, and radiopharmaceuticals, which have radioactive daughters, are generally avoided.